initially i had written with clientid but it showing error as null value. so i wrote clientId and my div doesn't have runat="server".
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Abhishek RanjanJun 10 '11 at 18:57

if you don't have runat="server", try using assigning an id to your div and address it in getElementById() by that ID. For example: <div id="fb-root">...</div> ...then document.getElementByid('fb-root')
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SpechtusAug 17 '11 at 21:22